1. Field of the Invention
The present invention relates generally to methods and apparatus for providing medical or surgical treatment using optical energy, and in particular to a method and apparatus for providing cosmetic and noncosmetic fractional treatment of tissue (e.g., skin) using optical radiation.
2. Description of the Related Art
Lasers can be used for cosmetic and noncosmetic treatment of tissue. For example, lasers are used in cosmetic dermatological procedures, such as skin resurfacing (including treatment of wrinkles), removal of pigmented lesions, treatment of vascular lesions, treatment of acne, treatment of acne scars, treatment of striae, etc.
The side effect profile of a dermatological laser treatment depends on a number of factors, such as the percentage of a skin area that is treated, the size of the treatment zones, shape of the treatment zone, and the character (e.g., ablative or nonablative, selective or nonselective, etc.) of the treatment that is delivered. Side effects can also result from variations within the patient population or the treatment environment. For example, the water content of a patient's skin can determine how deeply a water-absorbed wavelength of light penetrates into the skin. Other factors, such as the starting temperature of the skin and the temperature of the air can alter the effects of the laser on the skin and can affect the amount of pain perceived by the patient.
Fractional treatment can reduce some side effects relative to bulk treatment for a given level of treatment efficacy. The reduction in side effects is due in part to the improvement in predictability of the skin response that is possible with fractional treatment. Fractional treatment with a water-absorbed wavelength, for example, typically treats with very high local fluences that could not be tolerated in a bulk treatment. Skin can tolerate very high local fluences because tissue adjacent to each microscopic treatment region is spared and participates in the healing response of the wounded tissue. In fractional treatments, overtreatment and undertreatment typically results in a change in the size and shape of the lesion, but not a change in whether or not lesions occur. On the other hand, for bulk treatments, overtreatment may result in a lesion that scars an entire region of skin, while undertreatment may result in no lesion at all. Thus, through the use of very high local fluences, fractional treatments can reliably denature a desired portion of each illuminated region. Small variations in fractional treatment fluence or treatment conditions have less effect than corresponding variations would have in bulk treatment because fractional treatments can still reliably create clinically visible effects even if undertreated or overtreated.
Despite being more controlled than bulk treatments, fractional treatments still have unacceptable side effects that could be reduced by a device with improved control of lesion characteristics. For example, the side effect profile for many treatments is closely related to the percentage of cells at the dermal-epidermal junction (“DE junction”) of a tissue portion that are killed during treatment. For this reason, it can be desirable to limit the percentage of treated tissue in a region. However, the treatment coverage percentage is also related to treatment efficacy in many treatment types. To achieve the desired efficacy while maintaining an acceptable side effect risk profile, it is desirable to have good control over the lesion dimensional characteristics, such as treatment zone width and depth.
In other fractional treatments, the side effect profile is strongly dependent on the distance to healthy tissue in the plane of the DE junction. Cells at the DE junction that are adjacent to treatment zones help to repair the damage created by the laser at the treatment zone and the time required for repairing treatment zones is strongly dependent on the size and shape of the treatment zone at the DE junction. For this reason, it is frequently desirable to create treatment zones with a small lesion width.
Treatment efficacy can be improved in many cases by reaching deeper tissue within the skin. This is particularly true, for example, when treating dermal scar tissue that frequently comprises scar tissue deep within the reticular dermis. In order to have short healing times and deep treatment zones, treatment zones with large aspect ratios are desirable for certain conditions. To control the diameter of the lesion at the DE junction and the depth of treatment, it is beneficial to have control over the treatment zone characteristics.
Another example where control over lesion characteristics would yield improved treatment results is in controlling the character of the treatment zones. For example, some fractional treatments are desirably not semiablative in order to reduce the duration and intensity of downtime and associated wound care following fractional laser treatment. If there is no reason to promote the disruption of epidermal layers, then it is desirable to maintain an intact epidermis to avoid an increased risk of infection, such as through creation of an open wound. On the other hand, for some treatments, it is desirable for the treatment to be semi-ablative. For example, a semi-ablative treatment can allow permeation of topically applied substances that promote the healing of the treated tissue. Existing laser treatment systems typically provide treatment that is either semi-ablative or not semi-ablative and do not have the capability of switching modes between semi-ablative and non-semi-ablative fractional treatments. A system with such capability is desirable so that two systems do not need to be purchased to accomplish these two goals.
Thus, there is a need for a fractional optical treatment system that allows for improved and adjustable control over fractional lesion characteristics, such as treatment zone width and depth, treatment zone aspect ratio, and/or the degree of disruptiveness of microscopic treatment zones.